The present invention relates to a coated titanium anode for electrically highly loaded amalgam cells. The large amounts of chlorine gas developing on the anode in the high current loaded chlorine alkali electrolysis cells must be conducted away from the electrode range as quickly as possible, for energy conservation reasons.
As is well known, dwelling of the chlorine gas bubbles in the electrode range causes a substantial increase in cell voltage and a decrease of the current efficiency. This phenomenon, which has become known by the collective term "gas bubble effect", has, in past years, led to design changes in the graphite anode. For example, the horizontally arranged anode plates were provided with numerous slots and gas outlet openings, which did provide the desired effect up to current densities of D.sub.A = 10 kA/m.sup.2. However, the ceramic-like graphite anode of conventional design is beginning to be in the way of a further increase of the current density. Thus, at very high current densities, D.sub.A, greater than 10 kA/.sub.m 2, 2, it is difficult to remove the developed chlorine gas rapidly from the underside of the horizontal anode plate that itself is generously slotted and provided with very many gas outlet openings. The result is higher overvoltage on the graphite and greater voltage losses in the electrolyte enriched with chlorine gas bubbles. Moreover, with the number of slots and gas outlet openings, the inner resistance, the graphite loss and the sensitivity to transport of the anodes increase. It has been attempted to avoid this restriction by utilizing the graphite anode described in the German published application No. 2,029,640. This is an anode which is provided with a number of thin, vertical graphite plates, and in which the graphite plates are disposed transversely to the flow direction of the mercury density, corresponding in length to the cathode width, are slotted in a comb-like manner on their underside, and are provided on their upper side with sunk contact sleeves of anodic resistant material and, with the interconnection of trough-configured bellows of corrosion resistant elastomer, are connected with current distributor rails in such a manner that, with the exception of the graphite plate provided with sleeves, all current conducting parts remain excluded from the inside of the cell. Mercury cells equipped with this anode design and operated at current densities D.sub.A of 10 to 13 kA/.sub.m 2, really did let the voltage coefficient (k-value) drop below 0.11 (V.m.sup.2 /kA), which appeared impossible for amalgam cells provided with electrographite anodes. Thus, the anodes in German published application No. 2,029,640 took a step toward the objective of making possible the intensified chlorine alkali electrolysis in modern amalgam cells with low voltage. However, an exact analysis of the wearing out picture of this anode simultaneously led to the conclusion that voltage coefficients k smaller than 0.10 will remain unattainable in spite of further improvements of the graphite anode. The ridges (teeth) having a width of 12.5 mm and obtained by slotting the graphite plate having a thickness of 40 mm have become so pointed, as a result of the NaCl-electrolysis conducted at current densities between 10 and 13 kA/m.sup.2, that the anode part facing the Hg-cathode soon has only prismatic teeth of a width of the sharply processed profile of about 10 mm and a profile height of about 15 mm. The anode ranges still further removed from the cathode also showed unusually strong traces of electrochemical attack. This wearing down appearance at a simultaneously favorable k-value is exceedingly surprising, considering it is similar to the wearing down appearance, which in many cases is briefly termed "Haifischzahne" (shark teeth), of horizontal graphite anode plates in NaCl-electrolytes contaminated by bringing in alkali, which always provides for high cell voltage in spite of strong pressing of the anodes. It must be concluded therefrom that the anode construction for high loaded amalgam cells, according to German published application No. 2,029,640, on account of its better chlorine gas bubble discharge, will distribute the electric current far better than the conventional stamp-configured, horizontally arranged anode plate provided with slots and gas discharge openings. It is still very effective in electrolysis, conducted at high current densities, down to great surface ranges, that is to surface ranges that are 15 mm and more away from the ridge tips.
This important finding has to be taken fully into account in the development of an improved coated titanium anode or electrically highly loaded amalgam cells, particularly for utilization with anodic current densities D.sub.A greater than 10 kA/m.sup.2.
Metal anodes for amalgam cells are well known in which the active part consists of coated titanium in the form of perforated sheet or metal mesh. These anodes lack the height and large surface necessary for good current distribution.
Moreover, metal anodes are known in which the active coating is applied onto a horizontally disposed parallel row of round titanium rods, which are held together by uncoated transverse ribs. Such anodes of thin, round titanium rods also lack the height necessary for good current distribution. Although anodes produced from thick round rods do have the necessary height, the upper half of the activated rod surface, however, is in a position unfavorable relative to the mercury cathode, as it is in the shadow of the lower rod half. Current distribution to the activated surface of the upper rod half is further made difficult or, respectively, is entirely prevented by the extensive enrichment of the electrolyte, with chlorine gas bubbles in the narrow range of the gap between the round rods. Technical electrolysis tests with metal anodes, the active part of which consisted of completely coated titanium rods of a diameter greater than 5 mm, have shown, in the current density/range D.sub.A of 10 to 15 kA/m.sup.2, that the anode process takes place almost exclusively on the rod surface facing the mercury cathode, and that by increasing the spacing between the round rods current distribution could not be substantially improved.
Furthermore, metal anodes are known in which the active part consists of thin titanium strips, which are disposed either perpendicularly or at any other angle relative to the cathode, and which are provided, on at least a part of their surfaces, with a coating consisting of a metal of the platinum metal group. For example, British Pat. No. 1,076,973 discloses titanium anodes coated with platinum metal, the active part of which is present in the form of ribs or plates, respectively, which are disposed perpendicularly to the mercury cathode and parallel to the main direction of flow of the electric current and in the vicinity of which gas outlets or, respectively, gaps are disposed. The platinum metal coating is applied preferably or exclusively on the perpendicular area of the ribs or, respectively, plates, wherein at least half of the coated area and preferably at least 80% of the total coated area is arranged perpendicularly to the amalgam cathode, so that this coated area is about four times the area of the horizontal part of the coating. This measure is to counteract the danger of damage of the sensitive platinum metal coating by the amalgam in case of contact with the cathode. It also is to make possible operation of the anode at a spacing increased by about one mm from the cathode with the same cell voltage than with an anode produced from wire mesh, thus also reducing the danger of short circuiting. The important drawbacks of the suggested anodes include the small height of the active coating, amounting to only 2.54 mm, the unfavorable relation between the actual active surface of the anode and the projected surface of the anode at small distances with respect to the counterelectrode being at a maximum 2.03:1 and resulting from that construction of the anode, a complete absence of any active surface within the middle range and long distance range with respect to the amalgam cathode and, finally, the use of a collecting bar of titanium which has, because of its deep cross-slotting, only very small electrical conductivity, and the small actual active surface at small distances with respect to the counterelectrode.